feat(sisyphus-task): inherit parent model for categories and show fallback warning

- Change model priority: user override > parent model > category default
- Add ModelFallbackInfo to track model resolution type
- Show warning toast when category uses inherited or default model
- Add tests for model fallback info in task toast

src/plugin-handlers/config-handler.ts

@@ -154,7 +154,8 @@ export function createConfigHandler(deps: ConfigHandlerDeps) {
       };
 
       agentConfig["Sisyphus-Junior"] = createSisyphusJuniorAgentWithOverrides(
-        pluginConfig.agents?.["Sisyphus-Junior"]
+        pluginConfig.agents?.["Sisyphus-Junior"],
+        config.model as string | undefined
       );
 
       if (builderEnabled) {